Process for preparing synthetic resin molded articles having good antistatic property

ABSTRACT

A synthetic resin molded article having a good antistatic property, which including a synthetic resin substrate and a film of an antistatic polymer formed on the surface of the substrate. The antistatic polymer is prepared by polymerizing an anionic monomer represented by the formula (I) or a mixture of the anionic monomer and a monomer copolymerizable therewith: ##STR1## R 2 , R 3 , R 4  and R 5  independently represent an alkyl, aryl or aralkyl group which may have a substituent, n is 0, 1 or 2, B is an alkylene, arylene or aralkylene group which may have an ester bond, and R 6  is H or an alkyl group.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a synthetic resin molded article havinga good and durable antistatic property and a process for the preparationthereof.

(2) Description of the Related Art

At the present, many synthetic resin molded articles having many goodproperties are marketed, but since they generally have a high electricresistance and are easily charged with electricity by friction or thelike, they attract dust and dirt, and thus the appearance thereofbecomes poor.

As the means for imparting an antistatic property to synthetic resinmolded articles, there can be mentioned (1) internal addition of asurface active agent, (2) surface coating with a surface active agent,(3) surface coating with a silicon compound and (4) surface modificationby a plasma treatment. Of these methods, methods (3) and (4) are notpractical because of the high cost thereof, and methods (1) and (2) aregenerally adopted.

In the method of the internal addition of a surface active agent, sincea surface active agent is incorporated or dispersed in a syntheticresin-forming starting material before the polymerization or a syntheticresin before the molding, the preparation steps can be simplified, butto obtain a required antistatic property, it is generally necessary toincrease the amount of a surface active agent. This increase of theamount of the surface active agent added, however, tends to result in alowering of the mechanical strength of the synthetic resin, and theobtained antistatic property is easily lost when washed with water or byrubbing.

The method of coating the surface with a surface active agent isadvantageous in that the physical properties of the synthetic resin asthe substrate are not lowered and a good antistatic property can beobtained with a small amount of the surface active agent. On the otherhand, since the surface-coating step is necessary, an additional cost isrequired, and there is a danger that the beautiful appearance inherentlypossessed by a synthetic resin molded article will be lost. Moreover,the method has a problem in that the obtained antistatic property iseasily lost by water washing or rubbing.

The inventors previously disclosed a cationic antistatic polymer havinga quaternary ammonium base therein in Japanese Unexamined PatentPublication No. 63-108040, but this cationic polymer has a poor heatstability, and thus the polymer is adversely affected by heat.

SUMMARY OF THE INVENTION

In view of the foregoing, a primary object of the present invention toprovide a synthetic resin molded article having a good and durableantistatic property and retaining the inherent physical properties ofthe synthetic resin.

The inventors carried out research with a view to achieve the aboveobject, and as a result found that, by forming a film of a specificanionic polymer on the molding surface of a casting mold andpolymerizing a synthetic resin-forming starting material for a syntheticresin substrate by using this casting mold, a synthetic resin moldedarticle having a good and durable antistatic property is obtained.

More specifically, in accordance with the present invention, there isprovided a synthetic resin molded article having a good antistaticproperty, which comprises a synthetic resin substrate and a film of anantistatic polymer formed on the surface of the synthetic resinsubstrate, said antistatic polymer being formed by polymerizing ananionic monomer represented by the following general formula (I) or amixture comprising at least 20% by weight of said anionic monomer and upto 80% by weight of at least one monomer copolymerizable therewith:##STR2## wherein R₁ represents a hydrogen atom or a methyl group, A₁represents ##STR3## X represents a nitrogen atom or a phosphorus atom,R₂, R₃, R₄ and R₅ independently represent an alkyl, aryl or aralkylgroup having 1 to 18 carbon atoms which may have a substituent, n is 0,1 or 2, B represents an alkylene, arylene or aralkylene group having 1to 18 carbon atoms which may have an ester bond, and R6 represents ahydrogen atom or an alkyl group having 1 to 5 carbon atoms.

This molded article can be prepared by forming a film of the antistaticpolymer on the molding surface of a casting mold by polymerizing theabove-mentioned monomer or monomer mixture, casting a syntheticresin-forming starting material for the synthetic resin substrate intothe casting mold, polymerizing and curing the starting material totransfer the film to onto the synthetic resin substrate from the moldsurface and withdrawing the obtained molded article from the castingmold.

The anionic antistatic polymer has a good heat stability, and littledeterioration of the polymer occurs at the polymerizing and curing stepand heat processing step where the temperature is elevated, and fromthis viewpoint, the anionic antistatic polymer is advantageous overcationic antistatic polymers.

BRIEF DESCRIPTION OF THE DRAWING

The drawing illustrates an embodiment of the apparatus for thecontinuous preparation of the synthetic resin molded article of theinvention in the form of a methacrylic resin plate, which apparatus isprovided with a film-forming starting material-coating device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The anionic monomer used in the present invention is represented by thefollowing general formula (I): ##STR4##

As specific examples of the anionic monomer of general formula (I),there can be mentioned tetramethylammonium vinylsulfonate,benzyltrimethylammonium vinylsulfonate, tetraethylammoniumallylsulfonate, benzyltriethylammonium methallylsulfonate,methyltriethylolammonium sulfoethylmethacrylate, lauryltrimethylammoniumsulfoethylacrylate, tetramethylammonium2-acrylamido-2-methylpropanesulfonate, methyltriethylammonium2-methacrylamido-2-methylpropanesulfonate, methyltriethylolammoniumstyrenesulfonate/ethylene oxide adduct, tetrabutylammoniumα-methylstyrenesulfonate, tetraethylphosphonium vinylsulfonate,tetrabutylphosphonium vinylsulfonate, tetramethylolphosphoniumvinylsulfonate, tetrabutylphosphonium allylsulfonate,tetralaurylphosphonium methallylsulfonate, tributylmethylphosphoniumsulfoethylmethacrylate, triethylbutylphosphonium sulfoethylacrylate,tetrabutylphosphonium sulfopropylacrylamide, trimethylolbutylphosphoniumsulfopropylmethacrylamide, tetrabutylphosphonium styrenesulfonate,tetramethylolphosphonium styrenesulfonate and triethylmethylphosphoniumα-methylstyrenesulfonate.

Among these anionic monomers, an appropriate monomer is freely selectedaccording to the kind of the synthetic resin substrate used. Forexample, when a methyl methacrylate resin is used as the synthetic resinsubstrate, in view of the compatibility with the methyl methacrylateresin and the easy availability of the starting material, ammonium saltsof 2-acrylamido-2-methylpropanesulfonic acid, sulfoethylmethacrylic acidand sulfoethylacrylic acid are preferably used, and tetramethylammoniumsalts thereof are especially preferably used.

A monomer of general formula (I), in which at least one of R₂ through R₅is a hydrogen atom, is not preferable because the resistance againstthermal deterioration is lowered and discoloration occurs at thepolymerizing and curing step and heat processing step at which thetemperature is elevated, and the compatibility with the substratesynthetic resin and the adhesion to the synthetic resin substrate arereduced.

A known monomer can be used as the monomer copolymerizable with theanionic monomer of general formula (I). For example, there can bementioned methacrylic acid esters such as methyl methacrylate and ethylmethacrylate, acrylic acid esters such as methyl acrylate and ethylacrylate, unsaturated carboxylic acids such as acrylic acid andmethacrylic acid, acid anhydrides such as maleic anhydride and itaconicanhydride, maleimide derivatives such as N-phenylmaleimide, hydroxylgroup-containing monomers 2-hydroxyethyl acrylate and 2-hydroxypropylmethacrylate, nitrogen-containing monomers such as acrylamide andacrylonitrile, epoxy group-containing monomers such as allyl glycidylether and glycidyl acrylate, bifunctional monomers such as allylmethacrylate and allyl acrylate, and polymeric monomers such asmethacrylate-terminated polymethyl methacrylate, styryl-terminatedpolymethyl methacrylate, methacrylate-terminated polystyrene,methacrylate-terminated polyethylene glycol and methacrylate-terminatedacrylonitrile/styrene copolymer.

As the copolymerizable monomer, there are preferably used compoundsrepresented by the following general formula (II): ##STR5## wherein R₇represents a hydrogen atom or a methyl group, R₈ represents a hydrogenatom or an alkyl, aralkyl or aryl group having 1 to 18 carbon atoms,which has no copolymerizable functional group, A₂ represents an alkylenegroup having 2 to 4 carbon atoms, and m is an integer of from 0 to 500,and copolymerizable compounds having at least two unsaturated doublebonds. Especially preferably, at least two compounds selected from theforegoing two types are used in combination.

As the compound of general formula (II) in which m is 0, there can bementioned methyl acrylate, ethyl acrylate, methyl methacrylate, ethylmethacrylate, butyl methacrylate, lauryl methacrylate, 2-ethylhexylmethacrylate, stearyl methacrylate, benzyl methacrylate, phenylmethacrylate, cyclohexyl methacrylate and 2-hydroxyethyl methacrylate.

As the compound of general formula (II) in which m is from 2 to 500,there can be mentioned polyethylene glycol(4) monomethacrylate,polyethylene glycol(23) monoacrylate, polyethylene glycol(23)monomethacrylate, polyethylene glycol(300) monomethacrylate,polypropylene glycol(23) monomethacrylate, polybutylene glycol(23)monomethacrylate, polyethylene glycol(23) monomethacrylate monomethylether, polyethylene glycol(23) monomethacrylate monobutyl ether,polyethylene glycol(23) monomethacrylate monostearyl ether, polyethyleneglycol(23) monomethacrylate monophenyl ether, polyethylene glycol(23)monomethacrylate monobenzyl ether and polyethylene glycol(23)monomethacrylate mono-oleyl ether. Note, each parenthesized valueindicates the number of alkylene glycol units in the polyalkyleneglycol.

In view of the adhesion between the antistatic property-impartingcopolymer and the synthetic resin substrate, the copolymerizable monomeris preferably the same as the monomer constituting the substratesynthetic resin, or a monomer forming a synthetic resin having a goodcompatibility with the synthetic resin substrate.

For example, when the synthetic resin substrate is a polymer comprisingmethyl methacrylate as the main component, if a monomer of generalformula (II) in which m is 0 is used, a good adhesion is obtainedbetween the synthetic resin substrate and the antistaticproperty-imparting copolymer. Therefore, in this case, the film of theantistatic property-imparting copolymer is not left in the casting moldat the time of peeling, and a stable antistatic property can bemanifested regardless of kind of the casting mold used. If a monomer ofgeneral formula (II) in which m is from 2 to 500 is used, the releaseproperty of the synthetic resin molded article from the casting mold,especially the release property at a high temperature, is improved, andan antistatic synthetic resin molded article can be stably obtained.

As the copolymerizable monomer having at least two unsaturated doublebonds, there can be mentioned allyl acrylate, methallyl acrylate, vinylacrylate, allyl methacrylate, methallyl methacrylate, vinylmethacrylate, 1-chlorovinyl methacrylate, isopropenyl methacrylate,N-methacryloxymaleimide, ethylene glycol dimethacrylate, butanedioldimethacrylate, polyethylene glycol dimethacrylate, allyl vinyl ether,allyl vinyl ketone, trimethylolpropane trimethacrylate, pentaerythritoltetramethacrylate and triallyl cyanurate.

When the copolymerizable compound having at least two unsaturated doublebonds is used, the copolymer film of the present invention has acrosslinked portion and a residual double bond, which contribute to animprovement of the strength of the film per se, formation of a semi-IPNstructure of the synthetic resin substrate and formation of a chemicalbonding to the substrate by graft polymerization of the monomer to theresidual double bonding, and a result, the surface hardness of themolded article and the adhesion of the copolymer film to the syntheticresin substrate, i.e., the durable antistatic property, can be improved.

Especially, when one of the functional groups is a functional grouphaving a polymerizability lower than that of the methacryloyl group oracryloyl group, such as an allyl group, a methallyl group, a vinylidenegroup or a vinylene group, an unreacted double bond is left in thepolymeric antistatic agent and performs a graft polymerization duringthe polymerization for the synthetic resin substrates, and therefore, agood adhesion is obtained between the film of the antistaticproperty-imparting polymer and the synthetic resin substrate.

As the monomer copolymerizable with the anionic monomer represented bygeneral formula (I), when the synthetic resin substrate is a methylmethacrylate polymer, there is preferably used a combination of (a) acompound of general formula (II) in which m is from 2 to 0, (b) acompound of general formula (II) in which m is 0, especially methylmethacrylate, and (c) a copolymerizable compound having at least twounsaturated double bonds, especially allyl methacrylate or allylacrylate.

The antistatic polymer of the present invention comprises 20 to 100% byweight, preferably 20 to 80% by eight, of units derived from an anionicmonomer represented by general formula (I), and 0 to 80% by weight,preferably 20 to 80% by weight, of units derived from a copolymerizablemonomer. If the amount of the anionic monomer of general formula (I) issmaller than 20% by weight, a good antistatic property cannot be givento an obtained synthetic resin molded article, for example, amethacrylic resin cast plate. From the viewpoint of the adhesion of theantistatic polymer to the synthetic resin substrate, the copolymerizablemonomer other than the monomer of general formula (I) is preferably usedin an amount of at least 20% by weight.

When the synthetic resin substrate is a methyl methacrylate resin, anantistatic polymer comprising 20 to 70% by weight of units derived froman anionic monomer of the general formula (I), and as thecopolymerizable monomer, (a) 24.9 to 74.9% by weight of units derivedfrom a monomer of general formula (II) in which m is from 2 to 500, (b)5 to 55% by weight of units derived from a monomer of the generalformula (II) in which m is 0 and (c) 0.1 to 10% by weight of unitsderived from a copolymerizable compound having at least two unsaturateddouble bonds, is especially preferably used.

Preferably, the molecular weight of the antistatic polymer used in thepresent invention is at least 1,000. If the molecular weight of theantistatic polymer is lower than 1,000, a film having a good and durableantistatic performance is difficult to obtain.

A durable antistatic property is attained according to the presentinvention because the film of the antistatic polymer is integrated withthe synthetic resin substrate. More specifically, the film formed on thesurface of the casting molding is swollen with a synthetic resin-formingstarting material at the polymerization for the synthetic resinsubstrate, and in this state, the polymerization is advanced and thefilm is integrated with the as-polymerized surface portion of the moldedarticle obtained according to the present invention, the antistaticproperty thereof is not lowered, when washed with water or by rubbingand the molded article of the present invention is advantageous in thispoint over a product obtained according to the coating method using asurface active agent. Moreover, according to the present invention,since the film of the antistatic polymer is present only on the surfaceof the molded article, a good antistatic performance can be obtainedeven with a small amount of the antistatic polymer.

The starting material used for the synthetic resin substrate is notparticularly critical. For example, there can be mentioned methylmethacrylate, styrene and other polymerizable monomer, partialpolymerization products thereof, a polyol and a polyisocyanate, anoligomer having epoxy groups at both the terminals and a polyamine orpolyamide, an unsaturated polyester, a novolak polymer and abisoxadorine, a reactive silicone rubber oligomer, and a polycarbonatecyclic oligomer.

A methacrylic resin prepared from methyl methacrylate, a monomer mixturecomprising at least 50% by weight of methyl methacrylate and up to 50%by weight of at least one monomer copolymerizable therewith, or apartial polymerization product thereof is most preferably used as thesynthetic resin substrate.

As the monomer copolymerizable with methyl methacrylate, there can bementioned methacrylic acid esters such as ethyl methacrylate, butylmethacrylate and 2-ethylhexyl methacrylate, acrylic acid esters such asmethyl acrylate, ethyl acrylate, butyl acrylate and 2-ethylhexylacrylate, unsaturated carboxylic acids such as acrylic acid, methacrylicacid, maleic acid and itaconic acid, acid anhydrides such as maleicanhydride and itaconic anhydride, maleimide derivatives such asN-phenylmaleimide, N-cyclohexylmaleimide and N-t-butyl-maleimide,hydroxyl group-containing monomers such as 2-hydroxyethyl acrylate,2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate and2-hydroxypropyl methacrylate, nitrogen-containing monomers such asacrylamide, methacrylamide, acrylonitrile, methacrylonitrile, diacetoneacrylamide and dimethylaminoethyl methacrylate, epoxy group-containingmonomers such as allyl glycidyl ether, glycidyl acrylate and glycidylmethacrylate, styrene monomers such as styrene and α-methylstyrene, andcrosslinking agents such as ethylene glycol diacrylate, allyl acrylate,ethylene glycol dimethacrylate, allyl methacrylate, divinylbenzene andtrimethylolpropane triacrylate, although the copolymerizable monomersthat can be used are not limited to those exemplified above. The kindand amount of the copolymerizable monomer added are determined accordingto the intended synthetic resin molded article.

Additives such as colorants, release agents, ultraviolet absorbers, heatstabilizers and various fillers can be incorporated in the startingmaterial for the synthetic resin substrate used in the presentinvention.

As examples of the casting mold used in the present invention, there canbe mentioned those made of inorganic glasses such as tempered glass,metals such as stainless steel, aluminum and a chromium plated metal,and resins such as a polyester resin. The surface of the casting mold isgenerally a mirror-polished surface, but a surface which has beensubjected to a delustering treatment by forming fine undulations on thesurface can be used according to the intended object.

A method in which a solution of the copolymer in water and/or an organicsolvent is coated on the casting mold surface is a simple andadvantageous method for the formation of a film of the copolymer on thesurface of the casting mold. When the synthetic resin substrate is amethacrylic resin, in view of the adhesion to the synthetic resinsubstrate and the spreading of the solvent, there is especiallypreferably used a method in which methyl methacrylate or a mixturecomprising 50% by weight of methyl methacrylate and up to 50% by weightof a monoethylenically unsaturated monomer copolymerizable therewith, ora partial polymerization product thereof, is coated on the surface ofthe casting mold.

Additive components such as release agents, defoaming agents, levellingagents, monomers and crosslinking agents can be incorporated into theabove-mentioned solution or mixture, as long as the antistaticperformance of the film obtained from the solution or mixture, thepolymerizability of the synthetic resin substrate-forming startingmaterial and the physical properties of the synthetic resin substrate,are not lowered.

As the means for coating the above-mentioned solution or mixture, therecan be mentioned a spray coating method, a flow coating method, a barcoating method, and a dip coating method.

When a plate-shaped methacrylic resin molded article is preparedaccording to the present invention, from the viewpoint of theproductivity, there is preferably adopted a continuous casting processusing as a casting mold two confronting stainless steel endless beltseach having one mirror-polished surface, which are moved in the samedirection at the same speed.

The present invention will now be described in detail with reference tothe following examples, that by no means limit the scope of theinvention. In the examples, parts are by weight.

All of the electric properties of the samples were determined after theyhad been moisture-conditioned at a temperature of 20° C. and a relativehumidity of 65% for 1 day. The charge half-value time was measured underconditions of an applied voltage of 10,000 V, a sample-rotating speed of1,550 rpm, a voltage application time of 30 seconds, a measurementtemperature of 20° C., and a measurement relative humidity of 65%. Thevoltage of the sample at the application of the voltage was designatedas the initial voltage (V), and the time required for the voltage of thesample to fall from the initial voltage to 1/2 thereof was designated asthe charge half-value time (sec). The surface resistivity (Ω) after 1minute from the point of application of a voltage of 500 V was measuredat a measurement temperature of 20° C. and a measurement relativehumidity of 65%, as the surface resistivity by a high megohm meter(Model TR-8601 supplied by Takeda Riken).

The surface resistivity after water washing was measured by theabove-mentioned high megohm meter after the obtained plate had been cutinto a test piece having a size of 40 mm ×40 mm, and the test piece hadbeen strongly wiped 60 times with a gauze in running water.

The surface hardness was determined according to the pencil scratch testof JIS K-5400 (usual test methods for paints).

The transparency was evaluated based on the haze value by using anintegrating sphere haze meter (Model SEP-H-SS supplied by NipponSeimitsu Kogaku).

EXAMPLE 1

A glass flask equipped with stirring vanes was charged with 312.4 partsof 2-acrylamido-2-methylpropanesulfonic acid and 450 parts of methanol,and 550.3 parts of a 25% by weight solution of tetramethylammoniumhydroxide in methanol was added dropwise with violent stirring so thatthe temperature was held below 30° C. after the dropping, the mixturewas stirred for 30 minutes to obtain an anionic monomer (M-1), then, 4parts of azobisisobutyronitrile, 3 parts of n-octylmercaptan, 38 partsof methanol, and 423 parts of polyethylene glycol(23) monomethacrylatemonomethyl ether were added to the obtained anionic monomer (M-1)solution, and polymerization was carried out at 60° C. for 4 hours in anitrogen atmosphere. The reaction product was then vacuum-dried toobtain an antistatic polymer (P-1).

Then, 5 parts of the polymer (P-1) were dissolved in 95 parts of ethanolto prepare a film-forming starting material. On a mirror-polishedsurface of a stainless steel plate having a length of 600 mm, a width of450 mm and a thickness of 3 mm, the film-forming starting material wasspray-coated and dried. By using a pair of the thus-treated stainlesssteel plate and gaskets, a casting mold was constructed so that thethickness of the cast product was 3 mm. A synthetic resin-formingstarting material prepared by dissolving 0.05 part of2,2'-azobisisobutyronitrile in 100 parts of partially polymerized methylmethacrylate having a viscosity of 1,000 cP as determined at 20° C. anda polymerization conversion of 20%, and removing dissolved air under areduced pressure, was cast into the casting mold, and polymerization wascarried out at 60° C. for 10 hours and further at 110° C. for 4 hours.Then, the temperature was lowered to the normal temperature, and themolded product was parted from the casting mold.

The surface resistivity of the obtained methacrylic resin plate was9.2×10¹⁰ Ω, the charge half-value time was 1 second, and the haze valuewas 1.0%. The surface hardness was B as determined according to thepencil scratch test of JIS K-5400.

The obtained plate was washed with water and the antistatic propertyimmediately evaluated, and it was found that the surface resistivity ofthe obtained methacrylic resin plate was 3.5×10¹⁰ Ω and the chargehalf-value time was 1 second.

EXAMPLE 2

A glass flask equipped with stirring vanes was charged with 312.4 partsof 2-acrylamido-2-methylpropanesulfonic acid and 450 parts of methanol,and 1042.8 parts of a 40% by weight solution of tetrabutyl phosphoniumhydroxide in methanol was added dropwise with violent stirring so thatthe temperature was held below 30° C. After the dropping, the mixturewas stirred for 30 minutes to obtain a solution of an anionic monomer(M-2), and to the obtained anionic monomer (M-2) solution were added 4parts of azobisisobutyronitrile, 3 parts of n-octylmercaptan, 200 partsof methanol, and 702 parts of polyethylene glycol(23) monomethacrylatemonomethyl ether, and polymerization was carried out at 60° C. for 4hours in a nitrogen atmosphere. The polymerization product was directlyvacuum-dried to obtain an antistatic polymer (P-2), and a methacrylicresin plate was prepared in the same manner as described in Example 1 byusing the obtained polymer (P-2).

The surface resistivity of the obtained methacrylic resin plate was4.3×10¹⁰ Ω, the charge half-value time was 1 second, and the haze valuewas 1.0%. The surface hardness was B as determined according to thepencil scratch test of JIS K-5400.

The obtained plate was washed with water and the antistatic propertyimmediately evaluated, and it was found that the surface resistivity was9.5×10¹⁰ Ω and the charge half-value time was 1 second.

EXAMPLE 3

A glass flask equipped with stirring vanes was charged with 156.7 partsof 2-acrylamido-2-methylpropanesulfonic acid and 220 parts of methanol,and 276.1 parts of a 25% by weight solution of tetramethylammoniumhydroxide in methanol was added dropwise with violent stirring so thatthe temperature was held below 30° C. After the dropping, the mixturewas stirred for 30 minutes to obtain a solution of an anionic monomer(M-1), and to the obtained anionic monomer (M-1) solution were added 3parts of azobisisobutyronitrile, 2 parts of n-octylmercaptan, 80 partsof methanol, 283 parts of polyethylene glycol(23) monomethacrylatemonomethyl ether, 212 parts of methyl methacrylate and 18 parts of allylmethacrylate, and polymerization was carried out at 60° C. for 5 hoursin a nitrogen atmosphere to obtain a solution of an antistatic polymer(P-3). Then a methacrylic resin plate was prepared in the same manner asdescribed in Example 1 by using the obtained polymer (P-3).

The surface resistivity of the obtained methacrylic resin plate was5.8×10¹⁰ Ω, the charge half-value time was shorter than 1 second, andthe haze value was 1.0%. The surface hardness was 3H as determinedaccording to the pencil scratch test of JIS K-5400.

The obtained plate was washed with water and the antistatic propertyimmediately evaluated, and it was found that the surface resistivity was4.3×10¹⁰ Ω and the charge half-value time was shorter than 1 second.

EXAMPLE 4

A glass flask equipped with stirring vanes was charged with 156.7 partsof 2-acrylamido-2-methylpropanesulfonic acid and 220 parts of methanol,and 523.1 parts of a 40% by weight solution of tetrabutylphosphoniumhydroxide in methanol was added dropwise with stirring so that thetemperature was held below 30 minutes, to obtain a solution of ananionic monomer (M-2), and to the obtained anionic monomer (M-2)solution were added 3 parts of azobisisobutyronitrile, 2 parts ofn-octylmercaptan, 80 parts of methanol, 470 parts of polyethyleneglycol(23) monomethacrylate monomethyl ether, 352 parts of methylmethacrylate and 35 parts of allyl methacrylate, and polymerization wascarried out at 60° C. for 5 hours in a nitrogen atmosphere to obtain asolution of an antistatic polymer (P-4). A methacrylic resin plate wasprepared in the same manner as described in Example 1 by using theobtained polymer (P-4).

The surface resistivity of the obtained methacrylic resin plate was6.3×10¹⁰ Ω, the charge half-value time was 1 second, and the haze valuewas 1.0%. The surface hardness was 3H as determined according to thepencil scratch test of JIS K-5400.

The obtained plate was washed with water and the antistatic propertyimmediately evaluated, and it was found that the surface resistivity was6.5×10¹⁰ Ω and the charge half-value time was 1 second.

COMPARATIVE EXAMPLE 1

A methacrylic resin plate having a thickness of 3 mm was prepared in thesame manner as described in Example 1, except that a mirror-polishedstainless steel plate not treated with the antistatic polymer was used.

The surface resistivity of the plate was higher than 10¹⁶ Ω, the chargehalf-value time was longer than 120 seconds, the haze value was 1.0%,and the surface hardness was 3H.

EXAMPLE 5

A methacrylic resin plate having a thickness of 3 mm was prepared in thesame manner as described in Example 1 except that tempered glass sheetshaving a length of 600 mm, a width of 450 mm, and a thickness of 6 mmwas used as the casting mold. The surface resistivity of the obtainedresin plate was 7.8×10⁹ Ω, the charge half-value was 1 second, the hazevalue was 1.0%, and the surface hardness was HB.

After the water washing, the surface resistivity was 2.3×10¹⁰ Ω and thecharge half-value time was 1 second.

EXAMPLE 6

30 A laminate formed by bonding a polyester film (Lumilar supplied byToray; standard type having a thickness of 250 μm) to the surface of astainless steel plate having a length of 600 mm, a width of 450 mm, anda thickness of 3 mm was used as the casting mold, and a methacrylicresin plate having a thickness of 3 mm was prepared in the same manneras described in Example 1. The surface resistivity of the obtained resinplate was 7.2×10⁹ Ω, the charge half-value time was 1 second, and thesurface hardness was B.

After the water washing, the surface resistivity was 5.6×10¹⁰ Ω and thecharge half-value time was 1 second.

EXAMPLE 7

A methacrylic resin plate having a thickness of 3 mm was prepared in thesame manner as described in Example 1, except that a mixture of 2.0parts of polymer (P-1), 51.0 parts of methyl methacrylate and 47.0 partsof partially polymerized methyl methacrylate having a viscosity of 100cP and a polymerization conversion of 8% was coated as the film-formingstarting material on the casting mold by a bar coater. The surfaceresistivity of the obtained resin plate had a surface resistivity of2.3×10¹⁰ Ω, a charge half-value shorter than 1 second and a haze valueof 1.0%. The surface hardness was H as determined according to thepencil scratch test of JIS K-5400.

After the water washing, the surface resistivity was 2.5×10¹⁰ Ω and thecharge half-value time was shorter than 1 second.

EXAMPLE 8

A methacrylic resin plate was prepared in the same manner as describedin Example 2 except that 5 parts of ethylene glycol dimethacrylate wasused instead of allyl methacrylate. The surface resistivity of theobtained resin plate was 5.3×10¹⁰ Ω, the charge half-value time wasshorter than 1 second, and the haze value was 1.0%.

After the water washing, the surface resistivity was 6.4×10¹⁰ Ω and thecharge half-value time was shorter than shorter than 1 second. Thesurface hardness was 3H as determined according to the pencil scratchtest of JIS K-5400.

EXAMPLES 9 THROUGH 15

Polymers (P-5) through (P-11) shown in Table 2 were prepared in the samemanner as described in Example 1, by using the monomer (M-1) solution.Methacrylic resin plates having a thickness of 3 mm were prepared in thesame manner as described in Example 1, by using these polymers. Theevaluation results are shown in Table 2.

In Examples 9, 10 and 15, monomeric methyl methacrylate was used insteadof the partially polymerized methyl methacrylate as the substrateresin-forming starting material.

In Examples 9 and 15, the parting from the casting mold was verydifficult.

EXAMPLES 16 THROUGH 21

Monomers were prepared in the same manner as described in Example 1 or 2except that a sulfonic acid-containing monomer shown in Table 1 and aquaternary ammonium base or quaternary phosphonium base shown in Table 1were used.

                  TABLE 1                                                         ______________________________________                                        Mono-  Sulfonic Acid-Containing                                               mer    Monomer          Quaternary Base                                       ______________________________________                                        M-3    Allylsulfonic acid                                                                             Tetramethylammonium                                                           hydroxide                                             M-4    Sulfoethyl methacrylate                                                                        Lauryltrimethylammonium                                                       hydroxide                                             M-5    Styrenesulfonic acid                                                                           Benzyltrimethylammonium                                                       hydroxide                                             M-6    Allylsulfonic acid                                                                             Tetraethylphosphonium                                                         hydroxide                                             M-7    Sulfoethyl methacrylate                                                                        Benzyltriethylphospho-                                                        nium hydroxide                                        M-8    Styrenesulfonic acid                                                                           Tetrabutylphosphonium                                                         hydroxide                                             ______________________________________                                    

Polymers (P-12) through (P-17) shown in Table 2 were prepared in thesame manner as described in Example 1 or 2, by using solutions ofmonomers (M-3) through (M-8) shown in Table 1.

Methacrylic resin plates having a thickness of 3 mm were prepared in thesame manner as described in Example 1 by using these copolymers. Theevaluation results are shown in Table 2.

EXAMPLE 22

An apparatus for the continuous production of a methacrylic resin plateas shown in the accompanying drawing was used as the casting mold.

Referring to the accompanying drawings, belts 1 and 1'are endlessstainless steel belts having a mirror-polished surface, a width of 1.5 mand a thickness of 1 mm. The belts were moved at a speed of 2 m/min bydriving a main pulley 2, The initial tension on the belts was given byhydraulic cylinders arranged on pulleys 2 and 2' and set at 10 kg/mm² ofthe sectional area of the belts. Also reference numerals 3 and 3'represent pulleys.

Film-forming starting materials 5 and 5' comprising 2.0% by weight ofcopolymer (P-3), 96.0% by weight of methyl methacrylate and 2.0% byweight of methanol were coated on the mirror-polished surfaces of belts1 and 1' by roll coaters 6 and 6'.

The film-formed belts were arranged to confront each other, and bothside portions thereof were sealed by hollow pipe gaskets 15 of polyvinylchloride charged with a considerable amount of a plastizier. A syntheticresin substrate-forming starting material 14 comprising 100 parts ofpartially polymerized methyl methacrylate (the content of a polymerhaving an average degree of polymerization of 1,800 was 21%), 0.05 partof 2,2'-azobis(2,4-dimethylvaleronitrile) and 0.01 part of Tinuvin P wassupplied between the belts through a casting device by a metering pump.

The total length of the polymerization zone was 96 m. In the former parthaving a length of 66 m, the distance between the confronting surfacesof the belts as controlled by a plurality of idle rollers 4 and 4'arranged at intervals of 15 cm and warm water maintained at 80° C. wasspray-scattered on the outer surfaces of the belts. In the latter parthaving a length of 30 m, the belts were supported by idle rollersarranged at intervals of 1 m, and the cast material was heated at about130° C. by an infrared heater and then cooled. after the cooling, theproduct was peeled from the belts, and thus a methacrylic resin platehaving a thickness of 3 mm was continuously prepared.

The surface resistivity of the obtained resin plate was 2.0×10¹⁰ Ω, thecharge half-value time was shorter than 1 second, and the haze value was1.0%. The surface hardness was 3H as determined according to the pencilscratch test method of JIS K-5400.

After the water washing, the surface resistivity was 1.3×10¹⁰ Ω, and thecharge half-value time was shorter than 1 second.

COMPARATIVE EXAMPLES 2 AND 3

Polymers (P-18) and (P-19) shown in Table 2 were prepared in the samemanner as described in Example 1, by using the solution of monomer(M-1). Methacrylic resin plates having a thickness of 3 mm were preparedin the same manner as described in Example 1 by using these polymers.The evaluation results are shown in Table 2.

COMPARATIVE EXAMPLE 4

A methacrylic resin plate having a thickness of 3 mm was prepared in thesame manner as described in Example 1, except that a 10% by weightsolution of a coating type antistatic agent having a quaternary ammoniumbase (Statiside supplied by Analytical Chemical Laboratories) inmethanol was used as the film-forming starting material.

The surface resistivity of the obtained resin plate was 1.6×10⁹ Ω andthe half-value time was shorter than 1 second, but many fine undulationdefects were formed on the surface of the resin plate by a partialpeeling from the surface of the casting mold during the polymerization.

COMPARATIVE EXAMPLE 5

A glass flask equipped with stirring vanes was charged with 312.4 partsof 2-acrylamido-2-methylpropanesulfonic acid and 450 parts of methanol,and a mixture of 102.8 parts of 25% by weight aqueous ammonia and 240parts of methanol was added dropwise with violent stirring so that thetemperature was held below 30° C. The mixture was stirred for 30 minutesto obtain an anionic monomer (M-9) solution, and to the obtained anionicmonomer (M-9) solution were added 3.2 parts of azobisisobutyronitrile,2.4 parts of n-octyl-mercaptan, 30 parts of methanol and 360 parts ofpolyethylene glycol(23) monomethacrylate monomethyl ether, andpolymerization was carried out at 60° C. for 4 hours in a nitrogenatmosphere. After the polymerization, the polymerization product wasdirectly dried in vacuo to obtain an antistatic polymer (P-18).

A methacrylic resin plate was prepared in the same manner as describedin Example 1 by using the obtained polymer (P-18). Parting of the platefrom the stainless steel plate as the casting mold was not good, and apeeling phenomenon in which the antistatic polymer was left on thestainless steel plate was observed, and the surface of the methacrylicresin plate was slightly yellowed.

From the results of Comparative Example 5, it is seen that if countercations R₂ through R₅ in general formula (I) are hydrogen atoms, thecompatibility with the resin and the resistance against thermaldeterioration are lowered.

                                      TABLE 2                                     __________________________________________________________________________                                                        Surface                                                                       resistivity                           Monomers                         Surface                                                                              after                                                                               Hazer                               Amount      Amount      Amount                                                                             resistivity                                                                          washing                                                                             value                      Polymer                                                                            Kind                                                                              (parts)                                                                            Kind   (parts)                                                                            Kind   (parts)                                                                            (Ω)                                                                            (Ω)                                                                           (%)                 __________________________________________________________________________    Example 9                                                                            P-5  M-1 100  --     --   --     --   1.5 × 10.sup.10                                                                3.6                                                                           × 10.sup.12                                                                   1.2                 Example 10                                                                           P-6  M-1 80   PEG(23)                                                                              20   --     --   1.3 × 10.sup.10                                                                2.2                                                                                 1.0mes. 10.sup.1    Example 11                                                                           P-7  M-1 20   PEG(23)                                                                              40   Methyl 40   5.3 × 10.sup.11                                                                5.1                                                                                 1.0mes. 10.sup.1                                     methacrylate                                 Example 12                                                                           P-8  M-1 50   PEG(9) 50   --     --   2.3 × 10.sup.11                                                                8.5                                                                                 1.0mes. 10.sup.1    Example 13                                                                           P-9  M-1 50    PEG(500)                                                                            50   --     --   5.3 × 10.sup.10                                                                9.8                                                                                 1.0mes. 10.sup.1    Example 14                                                                           P-10 M-1 30   PEG(23)                                                                              40   Methyl 30   3.8 ×                                                                          4.5                                                                                 1.0mes. 10.sup.1                                     acrylate                                     Example 15                                                                           P-11 M-1 50   Methyl 50   --     --   1.2 × 10.sup.11                                                                9.8                                                                                 1.0mes. 10.sup.1                         methacrylate                                             Example 16                                                                           P-12 M-3 50   PEG(23)                                                                              50   --     --   2.5 × 10.sup.11                                                                7.3                                                                                 1.0mes. 10.sup.1    Example 17                                                                           P-13 M-4 50   PEG(23)                                                                              50   --     --   3.8 × 10.sup.11                                                                9.1                                                                                 1.0mes. 10.sup.1    Example 18                                                                           P-14 M-5 50   PEG(23)                                                                              50   --     --   7.3 × 10.sup.10                                                                3.6                                                                                 1.0mes. 10.sup.1    Example 19                                                                           P-15 M-6 50   PEG(23)                                                                              50   --     --   1.5 × 10.sup.10                                                                8.5                                                                                 1.0mes. 10.sup.1    Example 20                                                                           P-16 M-7 50   PEG(23)                                                                              50   --     --   8.7 × 10.sup.10                                                                2.2                                                                                 1.0mes. 10.sup.1    Example 21                                                                           P-17 M-8 50   PEG(23)                                                                              50   --     --   2.1 × 10.sup.10                                                                5.3                                                                                 1.0mes. 10.sup.1    Comparative                                                                          P-18 M-1 10   PEG(23)                                                                              90   --     --   5.3 × 10.sup.14                                                                9.3                                                                                 1.0mes. 10.sup.1    Example 2                                                                     Comparative                                                                          P-19 M-1 10   PEG(23)                                                                              10   Methyl 80   8.9 × 10.sup.14                                                                1.3                                                                                 1.0mes. 10.sup.1    Example 3                        methacrylate                                 __________________________________________________________________________     Note                                                                          PEG(23): polyethylene glycol(23) monomethacrylate monomethyl ether            PEG(9): polyethylene glycol(9) monomethacrylate monomethyl ether              PEG(500): polyethylene glycol(500) monomethacrylate monomethyl ether          Note, each parenthesized value indicates the number of alkylene glycol        units in the polyalkylene glycol.                                        

We claim:
 1. A process for the preparation of a synthetic resin moldedarticle having a good antistatic property, which comprises the stepsof:forming on the molding surface of a casting mold a film of anantistatic polymer by polymerizing an anionic monomer represented by thefollowing general formula (I) or a mixture comprising at least 20% byweight of said anionic monomer and up to 80% by weight of at least onemonomer copolymerizable therewith: ##STR6## wherein R₁ represents ahydrogen atom or a methyl group, A₁ represents --CH₂) )_(n), ##STR7## Xrepresents a nitrogen atom or a phosphorus atom, R₂, R₃, R₄ and R₅independently represent an alkylene, arylene or aralkylene group having1 to 18 carbon atoms which may have a substituent, n is 0, 1 or 2, Brepresents an alkylene, arylene or aralkylene group having 1 to 18carbon atoms which may have an ester bond, and R₆ represents a hydrogenatom or an alkyl group having 1 to 5 carbon atoms; casting a startingmaterial of a synthetic resin as the substrate into the casting mold;polymerizing said starting material to transfer said film to the surfaceof the substrate; and withdrawing the obtained molded article from thecasting mold.
 2. A process for the preparation of a synthetic resinmolded article according to claim 1, wherein X in the general formula(I) is a nitrogen atom.
 3. A process for the preparation of a syntheticresin molded article according to claim 1, wherein the starting materialof the synthetic resin substrate is methyl methacrylate or a monomermixture comprising at least 50% by weight of methyl methacrylate and upto 50% by weight of at least one monomer copolymerizable therewith, or apartial polymerization product thereof.
 4. A process for the preparationof a synthetic resin molded article according to claim 1, wherein thecopolymerizable monomer is selected from the group consisting ofcompounds represented by the general formula (II): ##STR8## wherein R₇represents a hydrogen atom or a methyl group, R₈ represents a hydrogenatom or an alkyl, aralkyl or aryl group having 1 to 18 carbon atoms,which has no copolymerizable functional group, A represents an alkylenegroup having 2 to 4 carbon atoms, and m is aninteger of from 0 to 500,and copolymerizable compounds having at least two unsaturated doublebonds.
 5. A process for the preparation a synthetic resin molded articleaccording to claim 1, wherein m in the general formula (II) is aninteger o from 2 to
 500. 6. A process for the preparation of a syntheticresin molded article according to claim 1, wherein the film of theantistatic polymer is formed by coating a solution of the antistaticpolymer in a liquid medium selected from the group consisting of waterand organic solvents on the surface of the casting mold.
 7. A processfor the preparation of a synthetic resin molded article according toclaim 1, wherein the film of the antistatic polymer is formed by coatingmethyl methacrylate or a monomer mixture comprising at least 50% byweight of methyl methacrylate and up to 50% by weight of at least onemonomer copolymerizable therewith, or a partial polymerization productthereof on the surface of the casting mold, followed by polymerizationof the coating.
 8. A process for the preparation of a synthetic resinmolded article according to claim 1, wherein the casting mold comprisestwo confronting stainless steel endless belts each having one surfacemirror-polished, and two gaskets; said belts and said gaskets beingmoved in the same direction at the same speed.